1. Field of the Invention
The present invention relates to a laser printer having a thermal fixing mechanism constituted by a heat roll, an oil coater, and the like.
2. Description of the Prior Art
Referring to FIGS. 1 through 3, a conventional laser printer will be described hereunder.
In the conventional printer with a thermal fixing mechanism, an electrostatic latent image is formed on a drum 1, the electrostatic latent image is developed with a developer 2 so as to form a toner image, and the toner image is transferred onto paper 4 by a transfer device 3. Then, the transferred toner image is preheated by a preheater 5 together with the paper 4, and pressured by means of a heat roll 6 and a pressure roll 7 so as to be thermally fixed. Being made of thermoplastic resin, the toner is fused to be fixed by the heat roll 6 heated to about 250.degree. C. At this time, in order to prevent generation of so-called offset that the toner partially adheres onto the heat roll 6, the surface of the heat roll 6 is coated at a point A with fuser oil, as disclosed in U. S. Pat. No. 4,182,263. The oil is reserved in a tank (not shown) so as to be supplied by a pump to an orifice 9 of an oil coater 8. A web 10 made of felt and having a thickness of about 1 mm is able to be impregnated with the oil. The oil coater 8 is urged by springs 11 so as to press the web 10 against the heat roll 6 at the point A, so that the oil is applied from the surface B of the oil coater 8 onto the heat roll 6 through the web 10. The heat roll 6 is rotating at a fixed speed in the direction of an arrow C in FIG. 2 so that the whole surface of the heat roll 6 is coated with the oil. The web 10 is pressed against the heat roll 6 at a point E of the oil coater 8 so that offset toner is removed from the surface of the heat roll 6 in case of occurrence of toner offset. Being channel-like shaped, the coater 8 is in contact with the web 10 also at a point F. The web 10 is moving relatively slowly from a supply roll 12 to a take-up roll 13, and tension T always acts on the web 10 because a frictional force acts between the heat roll 6 and the web 10. A relatively strong force is required for removing offset toner at the point E, while a weak force only capable of preventing occurrence of any gap between the oil coater 8, the web 10 and the heat roll 6 suffices at the point A.
In this configuration, however, these two forces cannot be controlled and the foregoing aimed function cannot be attained, for the two reasons as follows.
The first reason is that it is very difficult to make the oil coater 8 come in contact with the cylindrical heat roll 6 through the web 10 at the three points. The pressing forces at the three contacting points vary depending on various sizes such as the sizes 1.sub.1 and 1.sub.2 of the oil coater 8, the size in the vertically positional relation between the heat roll 6 and the oil coater 8, and the like.
The second reason is that the tension T acts on the web 10 as described above and the horizontal component of the tension T acts on the oil coater 8 at the points E and F to thereby push-down the oil coater 8 to the left. If the oil coater 8 moves down to the left, the oil coater 8 is separated from the web 10 at the point A, so that the oil is not normally applied onto the heat roll 6 at the point A to thereby generate a problem. The tension T is caused by the frictional force generated between the heat roll 6 rotating in the direction of the arrow C and the web 10 slowly moving in the direction of an arrow D in FIG. 2. The tension T scatters depending on the surface conditions of the web 10 and the heat roll 6, the oil adhesion condition on the heat roll 6, and the like, and it is difficult to correct the force of the coater 8 correspondingly to the tension T by using the springs 11 in advance. If the forces of the springs 11 are set to be large, the load of a motor (not shown) for rotating the heat roll 6 becomes large to cause another problem.
Further, in the case where a gap is generated between the oil coater 8 and the web 10 so that the oil falls down without being applied onto the heat roll 6 at the point A, there is little possibility that the oil can be applied from the surface of the web 10 onto the surface of the heat roll 6 sufficiently and uniformly at the point E, because the web 10 is carrying offset toner on the surface thereof at the point E.
There is a further conventional example in which in order to solve the foregoing problem that the heat roll is not sufficiently coated with oil for the reasons of the foregoing defects the supply quantity of oil is excessively increased and the surplus oil is recovered. In this conventional printer, however, there has been such a drawback that a surplus oil recovering system is complicated.